Organic light emitting diode having co-deposited emission layer with host, emitting dopant and auxiliary dopant and method of fabricating the same

ABSTRACT

Provided is an organic light emitting diode which can easily control color coordinates and improve a device&#39;s life span characteristic by using an auxiliary dopant having a higher band gap energy than that of a host, and preferably, having an absolute value of the highest occupied molecular orbital energy level equal to or higher than that of the host, or an absolute value of the lowest unoccupied molecular orbital energy level equal to or lower than that of the host. 
     The organic light emitting diode includes a first electrode, an emission layer disposed on the first electrode and including a host, an emitting dopant and an auxiliary dopant, and a second electrode disposed on the emission layer. Here, the auxiliary dopant has a higher band gap energy than the host. A method of fabricating the organic light emitting diode is provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No.10-2008-54283, filed Jun. 10, 2008, the disclosure of which is herebyincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present embodiments relate to an organic light emitting diode (OLED)and a method of fabricating the same. Some embodiments relate to an OLEDwhich uses an auxiliary dopant having a higher band gap energy than ahost of an emission layer, and preferably having an absolute value ofthe highest occupied molecular orbital energy level equal to or higherthan that of the host, or an absolute value of the lowest unoccupiedmolecular orbital energy level equal to or lower than that of the host,and thus can facilitate the control of color coordinates, and improve adevice's life span, and a method of fabricating the same.

2. Description of the Related Art

Organic light emitting diodes are self emissive displays, which are thinand light, and can have a simple structure fabricated in a simpleprocess, display a high quality picture with a wide viewing angle,implement good motion picture and high color purity, and have electricalcharacteristics of low power consumption and low driving voltage, whichare suitable for mobile displays.

Generally, OLEDs include a pixel electrode, an emission layer disposedon the pixel electrode, and a counter electrode disposed on the emissionlayer. In such an OLED, when a voltage is applied between the pixelelectrode and the counter electrode, holes and electrons are injectedinto the emission layer and recombined in the emission layer to generateexcitons, which transition from an excited state to a ground state,thereby emitting light.

The emission layer of the OLED includes a host and an emitting dopant.The host is generally contained in the emission layer at the highestproportion, and serves to facilitate the fabrication of the emissionlayer, and to support the structure of the emission layer. Further, whena voltage is applied between the pixel electrode and the counterelectrode, carriers are recombined in a host, and the dopant emits lightby the excited energy transferred from the host to the dopant.Meanwhile, the emitting dopant is a compound which is fluorescent orphosphorescent, and substantially emits light by excitation by aid ofthe excited energy transferred from the host.

An auxiliary dopant may be further included in the emission layer tocontrol charge movement in the host, other than the host and theemitting dopant. Conventionally, the auxiliary dopant is formed of amaterial having an emission spectrum in the same wavelength range as theemitting dopant, or has an energy level limited within an energy levelof the host. However, when the energy level of the auxiliary dopant islimited in that of the host, there is a limit to selection of theauxiliary dopant. In addition, it is difficult to control the colorcoordinates due to interference between energy levels of the host andthe dopants, or greater contribution of the auxiliary dopant to theemission than the emitting dopant. The present embodiments overcome theabove problems and provide additional advantages as well.

SUMMARY OF THE INVENTION

Aspects of the present embodiments provide an organic light emittingdiode which can facilitate control of color coordinates and improve adevice's life span, and a method of fabricating the same.

According to an embodiment, an organic light emitting diode includes: afirst electrode; an emission layer disposed on the first electrode andincluding a host, an emitting dopant and an auxiliary dopant; and asecond electrode disposed on the emission layer. Here, the auxiliarydopant has a higher band gap energy than the host.

According to another embodiment, a method of fabricating an organiclight emitting diode includes: forming a first electrode; forming anemission layer having a host, an emitting dopant and an auxiliary dopanton the first electrode; and forming a second electrode on the emissionlayer. Here, the auxiliary dopant has a higher band gap energy than thehost.

Additional aspects and/or advantages of the embodiments will be setforth in part in the description which follows and, in part, will beobvious from the description, or may be learned by practice of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and/or other aspects and advantages of the embodiments will becomeapparent and more readily appreciated from the following description ofthe embodiments, taken in conjunction with the accompanying drawings ofwhich:

FIGS. 1A and 1B are cross-sectional views of an organic light emittingdiode according to exemplary embodiments;

FIG. 2 is a graph of relative brightness versus life span for organiclight emitting diodes fabricated according to Experimental Examples 1and 2, and Comparative Example 1; and

FIG. 3 is a graph of relative brightness versus life span for organiclight emitting diodes fabricated according to Experimental Examples 3and 4, and Comparative Example 2.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the present embodiments,examples of which are shown in the accompanying drawings, wherein likereference numerals refer to the like elements throughout thespecification. The embodiments are described below in order to explainthe present embodiments by referring to the figures.

FIGS. 1A and 1B are cross-sectional views of an organic light emittingdiode according to exemplary embodiments.

Referring to FIG. 1A, first, a first electrode 100 is disposed on asubstrate (not illustrated). The substrate may be formed of, forexample, glass, plastic, or stainless steel. A thin film transistor (notillustrated) including a semiconductor layer, a gate electrode andsource and drain electrodes may be further formed on the substrate. Thethin film transistor is electrically connected to the first electrode100.

The first electrode 100 may be an anode, which may be a transparent orreflective electrode. When the first electrode 100 is a transparentelectrode, it may be formed of indium tin oxide (ITO), indium zinc oxide(IZO), tin oxide (TO), or zinc oxide (ZnO). When the first electrode 100is a reflective electrode, it may be formed in a stacked structure of areflective layer including, for example, silver (Ag), aluminum (Al),chromium (Cr), molybdenum (Mo), tungsten (W), titanium (Ti), gold (Au),palladium (Pd) or an alloy thereof, and a transparent layer of ITO, IZO,TO or ZnO, disposed on the reflective layer. The first electrode 100 maybe formed, for example, by sputtering, vapor phase deposition, ion beamdeposition, electron beam deposition or laser ablation.

An emission layer 110 is disposed on the first electrode 100. Theemission layer 110 includes a host, an emitting dopant and an auxiliarydopant.

In the present embodiments, the auxiliary dopant has a higher band gapenergy than the host. Also, the emitting dopant has a lower band gapenergy than the host. When the auxiliary dopant has a higher band gapenergy than the host, the flow of the energy from the host to theauxiliary dopant is inhibited, and thus the emission of the auxiliarydopant can be prevented. Accordingly, the auxiliary dopant has littleinfluence on color coordinates of the emission layer, so that the colorcoordinates of an organic light emitting diode can be easily controlled.

When the emission layer formed of the host and the emitting dopant has ahigher electron mobility than hole mobility, the auxiliary dopant may beformed of a material which has an absolute value of the lowestunoccupied molecular orbital (LUMO) energy level of the host equal to orlower than that of the host. The auxiliary dopant may serve to controlthe electron mobility of the emission layer, and reduce the differencebetween the hole mobility and the electron mobility of the emissionlayer. As a result, the recombination of electrons and holes in theemission layer can be increased, and thus the life span of the organiclight emitting diode is increased.

When the emission layer formed of the host and the emitting dopant has ahigher hole mobility than electron mobility, the auxiliary dopant may beformed of a material which has an absolute value of the highest occupiedmolecular orbital (HOMO) energy level equal to or higher than that ofthe host. In this case, the auxiliary dopant can serve to control thehole mobility of the emission layer, and reduce a difference between thehole mobility and the electron mobility of the emission layer. As aresult, the recombination of electrons and holes in the emission layeris increased, and thus the life span of the organic light emitting diodecan be increased.

The auxiliary dopant may be included in the emission layer 110 at aconcentration of from about 0.01 to about 30 wt %. When theconcentration of the auxiliary dopant is less than 0.01 wt %, theaddition of the auxiliary dopant may have little effect on the increasein life span of the diode. When the concentration of the auxiliarydopant is more than 30 wt %, the emission efficiency of the diode may bedegraded.

The auxiliary dopant may be co-deposited with the host and the emittingdopant to be included in the entire emission layer 110. Alternatively,the auxiliary dopant may be included only in a certain region in athickness direction of the emission layer 110. For example, the emissionlayer 110 can have a stacked structure of a first layer formed byco-depositing the host and the emitting dopant and a second layer formedby co-depositing the host, the emitting dopant and the auxiliary dopant,so that the auxiliary dopant may be included only in a certain region inthe thickness direction of the emission layer 110. Alternatively, theemission layer 110 can have a stacked structure of a first layer formedby co-depositing the host and the emitting dopant, and a second layerformed by co-depositing the host and the auxiliary dopant, so that theauxiliary dopant may be included only in a certain region in thethickness direction of the emission layer 110.

The host, the emitting dopant and the auxiliary dopant may appropriatelyemploy many different materials. For example, the host materials mayinclude 4,4′-N,N′-dicarbazole-biphenyl (CBP),bis(2-methyl-8-quinolinato)-4-phenylphenolate aluminum (BAlq),2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP),N,N′-dicabazolyl-1,4-dimethene-benzene (DCB) or rubrene. Examples of thematerials for the emitting dopant and the auxiliary dopant include4,4′-bis(2,2′-diphenyl vinyl)-1,1′-biphenyl (DPVBi), distyrylaminederivatives, pyrene derivatives, perylene derivatives, distyrylbiphenyl(DSBP) derivatives,10-(1,3-benzodiazole-2-yl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-pyrano(2,3-f)pyrido(3,2,1-ij)quinoline-11-one(C545T), quinacridone derivatives, tris(2-phenylpyridine)iridium(Ir(PPy)₃), PQIr, Btp₂Ir (acac),4-(dicyanomethylene)₂-tert-buthyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyrane(DCJTB),4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)4H-pyrane (DCM),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphyrin-platinum (PtOEP)complex, Ir(piq)₂ (acac), RD3 (Eastman Kodak Co, Rochester, N.Y.),

and the like.

A second electrode 120 is disposed on the emission layer 110. The secondelectrode 120 may be a cathode, or a transparent or reflectiveelectrode. When the second electrode 120 is a transparent electrode, itmay be formed as thin as possible so that light can pass through using,for example, Mg, Ca, Al, Ag or alloys thereof, which are conductivemetals having a low work function. When the second electrode 120 is areflective electrode, it may be formed more thickly so as to reflectlight. The second electrode 120 may be formed, for example, bysputtering, vapor phase deposition, ion beam deposition, electron beamdeposition or laser ablation.

Referring to FIG. 1B, when the first electrode 100 is an anode and thesecond electrode 120 is a cathode, at least one of a hole injectionlayer 130, a hole transport layer 140 and an electron blocking layer 150may be disposed between the first electrode 100 and the emission layer110, and at least one of a hole blocking layer 160, an electrontransport layer 170 and an electron injection layer 180 may be furtherdisposed between the emission layer 110 and the second electrode 120.

The hole injection layer 130 may be formed of an aryl amine-basedcompound, a phthalocyanine compound or starburst amines, for example,4,4,4-tris(3-methylphenylamino)triphenylamino (m-MTDATA),1,3,5-tris[4-(3-methylphenylamino)phenyl]benzene (m-MTDATB) or copperphthalocyanine (CuPc). The hole transport layer 140 may be formed ofarylene diamine derivatives, a starburst compound, biphenyldiaminederivatives having a spiro group or a trapezoidal compound, for example,N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD),N,N′-di(naphthalene-1-yl)-N,N′-diphenyl benzidine (α-NPD), or4,4′-bis(1-naphthylphenylamino)biphenyl (NPB). The electron blockinglayer 150 may be formed of BAlq, BCP, CF—X,3-(4-t-butylphenyl)-4-phenyl-5-(4-biphenyl)-1,2,4-triazole (TAZ) orspiro-TAZ and the like.

In addition, the hole blocking layer 160 may be formed of2-(4-biphenyl)-5-(4-tert-butylphenyl)-1,3,4-oxydiazole (PBD), spiro-PBDor TAZ, and the electron transport layer 170 may be formed of TAZ, PBD,spiro-PBD, Alq3, BAlq, SAlq or TYE 704 (Toyo Ink Mfg. Co. Ltd., Tokyo,Japan). The electron injection layer 180 may be formed of LiF, Gacomplex, Liq or CsF.

The hole injection layer 130, the hole transport layer 140, the electronblocking layer 150, the hole blocking layer 160, the electron transportlayer 170 and the electron injection layer 180 may be formed by thermalvacuum deposition, vapor phase deposition, spin coating, dip coating,doctor blading, inkjet printing or thermal transfer.

In the present embodiments, the auxiliary dopant is formed of a materialhaving a higher band gap energy than that of the host, so that emissionof the auxiliary dopant can be inhibited and thus color coordinates canbe easily controlled. The hole or electron mobility in the emissionlayer is controlled using an auxiliary dopant having an absolute valueof the LUMO energy level equal to or lower than that of the host, or anabsolute value of the HOMO energy level equal to or higher than that ofthe host, so that the recombination of electrons and holes in theemission layer 110 can be increased, and thus the life spancharacteristics of the organic light emitting diode can be improved.

Hereinafter, experimental examples and comparative examples will beprovided to help understanding of the present embodiments. However,these examples are only to help understanding of the presentembodiments, not to limit the present embodiments.

Experimental Example 1

A first electrode was formed to a thickness of 1000 Å using indium tinoxide (ITO). Subsequently, a hole transport layer was formed to athickness of 1000 Å using NPB on the first electrode. A red emissionlayer including rubrene as a host, 0.3 wt % RD3 (Kodak) as an emittingdopant, and 0.3 wt %

as an auxiliary dopant was formed on the hole transport layer. Theemission layer was formed to a thickness of 400 Å. An electron transportlayer was formed to a thickness of 250 Å using TYE 704 (Toyo Ink Mfg.Co. Ltd., Tokyo, Japan) on the emission layer. An electron injectionlayer was formed to a thickness of 50 Å using LiF on the electrontransport layer. A second electrode was formed to a thickness of 1500 Åusing Al on the electron injection layer.

Experimental Example 2

The process described above for Experimental Example 1 was carried outexcept that

was included in the emission layer as an auxiliary dopant.

Comparative Example 1

The process described above for Experimental Example 1 was carried outexcept that an auxiliary dopant was not included in the emission layer.

Table 1 shows LUMO and HOMO energy levels and band gap energies formaterials used in Experimental Examples 1 and 2 and Comparative Example1, and Table 2 shows hole mobility, electron mobility, driving voltage,efficiency and color coordinates for emission layers of organic lightemitting diodes fabricated according to Exemplary examples 1 and 2 andComparative Example 1. In addition, FIG. 2 is a life span graph for adiode, in which a horizontal axis is life span (hr), and a vertical axisis relative brightness (%).

TABLE 1 LUMO energy HOMO energy Band gap level (eV) level (eV) energy(eV) Host −3.2 −5.4 2.2 Emitting dopant −3.4 −5.5 2.1 Auxiliary dopantin −3.04 −5.5 2.46 E. Example 1 Auxiliary dopant in −2.97 −5.6 2.63 E.Example 2

TABLE 2 Electron Hole mobility mobility Driving Efficiency Colorcoordinates (cm²/V · s) (cm²/V · s) voltage (V) (cd/A) (CIE x, CIE y) E.Example 1 7.77216 * 10⁻⁷ 4.47519 * 10⁻⁶ 6.4 5.2 (0.658, 0.341) E.Example 2 3.82615 * 10⁻⁶ 7.30065 * 10⁻⁶ 6.1 5.4 (0.663, 0.336) C.Example 1 2.37209 * 10⁻⁴ 1.42814 * 10⁻⁵ 6.0 5.3 (0.660, 0.338)

Referring to Table 2, the diodes according to Experimental Examples 1and 2 showed almost the same characteristics in driving voltage andefficiency as that according to Comparative Example 1. Particularly, itcan be noted that there was no significant difference in colorcoordinates, according to which the auxiliary dopant did not participatein emission in Experimental Examples 1 and 2.

In Experimental Examples 1 and 2, the auxiliary dopant has a lowerabsolute value of the LUMO energy level than that of the host, and ahigher absolute value of the HOMO energy level than that of the host.Referring to Table 2, it can be noted that the hole mobility and theelectron mobility were controlled by the addition of the auxiliarydopant in Comparative Example 1. Thus, it can be noted that a differencebetween the electron mobility and the hole mobility in the emissionlayer was reduced in the diodes according to Experimental Examples 1 and2, and as shown in FIG. 2, the life span was significantly improved,compared to Comparative Example 1.

Experimental Example 3

A first electrode was formed to a thickness of 1000 Å using ITO.Subsequently, a hole transport layer was formed to a thickness of 1000 Åusing NPB on the first electrode. A red emission layer was formed bystacking a 400 Å-thick first layer including rubrene as a host and 0.3wt % RD3 (Kodak) as an emitting dopant on the hole transport layer, anda 150 Å-thick second layer including the host, the emitting dopant and0.3 wt %

as an auxiliary dopant on the first layer. An electron transport layerwas formed to a thickness of 250 Å using TYE 704 (Toyo Ink) on theemission layer. An electron injection layer was formed to a thickness of50 Å using LiF on the electron transport layer. A second electrode wasformed to a thickness of 1500 Å using Al on the electron injectionlayer.

Experimental Example 4

The process described above for Experimental Example 3 was carried outexcept that a second layer of an emission layer included only a host andan auxiliary dopant.

Comparative Example 2

The experiment was performed under the same conditions as ExperimentalExample 3 except that a second layer was not formed, and only anemission layer including a host and an emitting dopant was formed to athickness of 550 Å,

Table 3 shows driving voltage, efficiency and an x value of colorcoordinates for diodes according to Experimental Examples 3 and 4 andComparative Example 2. Table 3 shows a life span graph for the diode, inwhich a horizontal axis is life span (hr), and a vertical axis isrelative brightness (%).

TABLE 3 Driving voltage Efficiency Color coordinates (V) (cd/A) (CIE x)E. Example 3 6.9 5.4 0.661 E. Example 4 6.1 5.5 0.650 C. Example 2 6.45.5 0.662

Referring to Table 3 and FIG. 3, although the auxiliary dopant isincluded only in a certain region of the emission layer in a thicknessdirection, the diode shows almost the same characteristics in drivingvoltage, efficiency and color purity as that according to ComparativeExample 2, but its life span characteristic is significantly improved.

In the present embodiments, an organic light emitting diode can easilycontrol color coordinates and improve a life span characteristic byusing an auxiliary dopant having a higher band gap energy than that of ahost, and preferably, having an absolute value of the HOMO energy levelequal to or higher than that of the host, or an absolute value of theLUMO energy level equal to or lower than that of the host.

Although a few embodiments have been shown and described, it would beappreciated by those skilled in the art that changes may be made in thisembodiment without departing from the principles and spirit of theembodiments, the scope of which is defined in the claims and theirequivalents.

What is claimed is:
 1. An organic light emitting diode (OLED),comprising: a first electrode; an emission layer disposed on the firstelectrode and including a host, an emitting dopant and an auxiliarydopant; and a second electrode disposed on the emission layer, whereinthe auxiliary dopant has a higher band gap energy than the host, whereinthe emission layer has a lower hole mobility than that of a layer formedof only the host and the emitting dopant, and the auxiliary dopant hasan absolute value of the highest occupied molecular orbital energy levelequal to or higher than that of the host, and wherein the auxiliarydopant is included only in a certain region and not other regions in athickness direction of the emission layer; and wherein the emissionlayer includes a first layer not including the auxiliary dopant and asecond layer including the auxiliary dopant.
 2. The OLED according toclaim 1, wherein the emission layer is formed in a stacked structureincluding the first layer in which the host and the emitting dopant areco-deposited, and the second layer in which the host, the emittingdopant and the auxiliary dopant are co-deposited.
 3. The OLED accordingto claim 1, wherein the emission layer is formed in a stacked structureincluding the first layer in which the host and the emitting dopant areco-deposited, and the second layer in which the host and the auxiliarydopant are co-deposited.
 4. The OLED according to claim 1, wherein theemitting dopant has a lower band gap energy than the host.
 5. A methodof fabricating an organic light emitting diode (OLED), comprising:forming a first electrode; forming an emission layer including a host,an emitting dopant and an auxiliary dopant on the first electrode; andforming a second electrode on the emission layer, wherein the auxiliarydopant has a higher band gap energy than the host, wherein the emissionlayer has a lower hole mobility than that of a layer formed of only thehost and the emitting dopant, and the auxiliary dopant has an absolutevalue of the highest occupied molecular orbital energy level equal to orhigher than that of the host, and wherein the auxiliary dopant isincluded only in a certain region and not in other regions in athickness direction of the emission layer; and wherein the emissionlayer includes a first layer not including the auxiliary dopant and asecond layer including the auxiliary dopant.
 6. The method according toclaim 5, wherein the emission layer includes the first layer formed byco-depositing the host and the emitting dopant, and the second layerformed by co-depositing the host, the emitting dopant and the auxiliarydopant.
 7. The method according to claim 5, wherein the emission layerincludes the first layer formed by co-depositing the host and theemitting dopant, and the second layer formed by co-depositing the hostand the auxiliary dopant.
 8. An organic light emitting diode (OLED),comprising: a first electrode; an emission layer disposed on the firstelectrode and including a host, an emitting dopant and an auxiliarydopant; and a second electrode disposed on the emission layer, whereinthe auxiliary dopant has a higher band gap energy than the host, whereinthe auxiliary dopant is included only in a certain region and not otherregions in a thickness direction of the emission layer; and wherein theemission layer has a lower electron mobility than that of a layer formedof only the host and the emitting dopant, and the auxiliary dopant hasan absolute value of the lowest unoccupied molecular orbital energylevel equal to or lower than that of the host; and wherein the emissionlayer includes a first layer not including the auxiliary dopant and asecond layer including the auxiliary dopant.
 9. The OLED according toclaim 8, wherein the emission layer is formed in a stacked structureincluding the first layer in which the host and the emitting dopant areco-deposited, and the second layer in which the host, the emittingdopant and the auxiliary dopant are co-deposited.
 10. The OLED accordingto claim 8, wherein the emission layer is formed in a stacked structureincluding the first layer in which the host and the emitting dopant areco-deposited, and the second layer in which the host and the auxiliarydopant are co-deposited.
 11. The OLED according to claim 8, wherein theemitting dopant has a lower band gap energy than the host.
 12. A methodof fabricating an organic light emitting diode (OLED), comprising:forming a first electrode; forming an emission layer including a host,an emitting dopant and an auxiliary dopant on the first electrode; andforming a second electrode on the emission layer, wherein the auxiliarydopant has a higher band gap energy than the host, wherein the auxiliarydopant is included only in a certain region and not other regions in athickness direction of the emission layer; and wherein the emissionlayer has a lower electron mobility than that of a layer formed of onlythe host and the emitting dopant, and the auxiliary dopant has anabsolute value of the lowest unoccupied molecular orbital energy levelequal to or lower than that of the host; and wherein the emission layerincludes a first layer not including the auxiliary dopant and a secondlayer including the auxiliary dopant.
 13. The method according to claim12, wherein the emission layer includes the first layer formed byco-depositing the host and the emitting dopant, and the second layerformed by co-depositing the host, the emitting dopant and the auxiliarydopant.
 14. The method according to claim 12, wherein the emission layerincludes the first layer formed by co-depositing the host and theemitting dopant, and the second layer formed by co-depositing the hostand the auxiliary dopant.